1. Field of the Invention
This invention relates to a self-contained pre-manufactured air unit for installation on a roof top of a building. This unit has three connection points for ductwork. One connection point is used to supply air to the building and the second connection point is used to return air from the building through an air circulation system that includes a blower. The third connection point is also part of the unit, but is connected independently of the blower and is used to relieve pressure within the building. The unit may have a heating coil, a cooling coil or a heating/cooling coil and an air inlet for ambient air.
2. Description of the Prior Art
Self-contained pre-manufactured roof top heating/cooling units are known in which the units have two duct connections points, one duct connection point to supply air to the interior of a building and the second to receive return air from the building. Relief and outdoor air intake damper assemblies are located within the unit at the point that the return air enters the unit. The relief damper is pressure operated and relieves some of the return air to the outside of the unit. The outdoor intake damper assembly introduces outdoor air into the supply duct to the building. The units have a main fan or blower to force air through the unit and cause air flow through the relief/outdoor air intake damper assemblies. The units can cause a pressure buildup in the building as buildings are now constructed to be airtight. It is important to introduce fresh air into the buildings for health reasons. The pressure buildup can cause exit doors in the building, some of which are designed to close automatically when someone enters or exits the building, to remain open, thereby creating a security risk. Further, the pressure buildup can cause damage to the building. It is known that the pressure buildup from the heating/cooling units can sometimes cause the roof of a building to partially separate from the building. The pressure buildup is often caused by the pressure operated relief damper within the housing not opening properly as the damper is subject to opposing forces caused by operation of the main fan. The damper is usually designed to open outward, but the intake pressure created by the fan can also pull the damper inward to the closed position. Also, when the damper is exposed to the weather, the wind can blow against the damper from outside to keep it in the closed position or the damper can be fixed in the closed position by a buildup of snow or ice.
In some previous roof top air units, attempts have been made to relieve air pressure within buildings by placing a secondary fan on an exhaust vent for some of the return air from the housing. The secondary fan often does not work very well to relieve pressure within the building because the secondary fan is opposed by the main fan or blower within the housing. The main fan or blower forces air into the building and causes return air to exit from the building. When the main fan or blower is operating, the damper in the exhaust vent can fail to open satisfactorily or will not operate properly as the secondary fan is pulling in one direction and that the main fan or blower is pulling the damper in the opposite direction.